(a) Field of the Invention:
This invention relates to an improved process for producing porous films. More particularly, it relates to a process for producing porous films which comprises blending a polyolefin resin with barium sulfate as a filler, melting the resulting resin composition and forming it into a film, and then stretching the film at least uniaxially.
(b) Description of the Prior Art:
Conventionally, there are a large number of well-known processes for producing porous films by forming a resin composition comprising a polyolefin resin and any of various noncompatible fillers into a film and then stretching this film. For example, Japanese Patent Laid-Open Nos. 47334/'82 and 203520/'82 disclose a process for producing porous films which comprises melting a resin composition obtained by blending a polyolefin resin with a filler and liquid rubber or a hydroxylated poly-saturated-hydrocarbon, forming the molten resin composition into a sheet or film, and then stretching this sheet or film. Moreover, Japanese Patent Laid-Open No. 15538/'83 discloses a process for producing porous films which comprises melting a resin composition obtained by blending a linear low-density polyethylene resin with a filler and a liquid or waxy hydrocarbon polymer, forming the molten resin composition into a sheet or film, and then stretching this sheet or film. However, the films produced by these processes are disadvantageous in that they exhibit surface tackiness due to the aforesaid component contained in addition to the polyolefin resin and the filler and in that they can only be practically used in relatively large thickness because of their low mechanical strength.
Furthermore, it is described in Japanese Patent Laid-Open No. 149303/'83 that such porous films can be used as a leakproof sheet in disposable diapers. Porous films for use as the leakproof sheet of a disposable diaper are produced by blending 100 parts by weight of a polyolefin resin with 28 to 200 parts by weight of a filler and 10 to 70 parts by weight of a liquid or waxy hydrocarbon polymer, forming the resulting resin composition into a film, and then stretching this film at least uniaxially by a factor of 1.2 or greater. However, this process for producing porous films has the disadvantages that some types of fillers give poor stretchability and hence fail to provide fully uniform pores and that the resulting film tends to produce a disagreeable noise. Moreover, the concurrent use of a hydrocarbon polymer makes this process unsatisfactory because the hydrocarbon polymer tends to bloom to the film surface and causes a sticky sensation.
Such porous films are also useful as a leakproof sheet in sanitary napkins. Conventionally, a sheet of paper which has been rendered liquidimpermeable by means of a synthetic resin such as polyethylene has been used for this purpose. However, the resulting sanitary napkins have the disadvantage of causing a disagreeable sensation during prolonged use because of their lack of permeability to water vapor.